1. Field of Invention
Aspects of the invention pertain to surgical instruments, and more particularly to wrist mechanisms for minimally invasive instruments.
2. Art
In a telerobotic surgical system, wristed surgical end effectors on minimally invasive surgical instruments provide one or more degrees of freedom (DOFs) at a surgical site within a patient. For example, FIG. 1 is a diagrammatic view of a typical minimally invasive surgical instrument used by the da Vinci® Surgical System, manufactured by Intuitive Surgical, Inc., Sunnyvale, Calif. The instrument includes a force transmission mechanism 2 that is removably coupled to a robotic manipulator arm in the surgical system (see also FIG. 6 and the associated description below). Rotational forces from servomotor actuators in the manipulator arm engage components in transmission mechanism 2, which in turn transmits the forces to cables or cable/hypotube combinations that run through shaft 4. A surgical end effector 6 (e.g., grasper, scissors, retractor, stabilizer, cautery implement, and the like) is positioned at the distal (towards the surgical site) end of shaft 4. Wrist mechanism 8 provides DOFs for end effector 6. For reference purposes herein, locations closer to the surgical site may be referred to as distal, and locations farther from the surgical site may be referred to as proximal. Details of illustrative instrument implementations, including examples of transmission mechanisms, wrists, and end effectors, are found in, e.g., U.S. Pat. No. 6,394,998 B1 (filed Sep. 17, 1999), which is incorporated by reference. A brief summary is provided with reference to FIGS. 2A and 2B.
FIG. 2A is an illustrative diagrammatic elevation view of a portion of a wrist mechanism 8 for a minimally invasive surgical instrument. A clevis 9 (illustrated in dashed line), which may be referred to herein as a proximal clevis in the instrument, is positioned at the distal end of shaft 4. A clevis link 10 is positioned in and is held by the proximal clevis 9. Clevis link 10 includes a pulley portion 12 at a proximal end and a clevis portion 14 at a distal end. Clevis portion 14 may be referred to herein as a distal clevis in the instrument. Clevis portion 14 holds one or more pivoting members. In FIG. 2A, two jaw members 16a,16b are shown.
Two illustrative cables 18a,18b are used to move clevis link 10 with reference to shaft 4. The term “cable” is broadly used herein to mean any tendon-like component (e.g., wire, twisted wire cable, etc.). As shown in FIG. 2A, cable 18a extends through and out of the distal end of shaft 4 and is coupled to the “top” of clevis link 10. Likewise, cable 18b extends through and out of the distal end of shaft 4 and is coupled to the “bottom” of clevis link 10. Consequently, clevis link 10 pivots around axis 20 as tensile forces are alternatively applied and removed from cables 18a, 18b. The pivoting movement of clevis link 10 around axis 20, as indicated by the directional arrows, is arbitrarily referred to herein as pitch (motion into and out of the page is therefore arbitrarily referred to herein as yaw). As clevis link 10 rotates around axis 20, the cables 18a,18b wrap around the grooved circumference of pulley portion 12. As a result, a constant moment arm r1 is created between axis 20 and the point on pulley portion 12 at which a cable in tension is tangent.
FIG. 2B shows an illustrative implementation of the wrist mechanism described above. FIG. 2B shows the distal end of a “Long Tip Forceps” instrument (model nos. 400048 or 420048) used with da Vinci® Surgical Systems. The proximal clevis 22 is clearly seen at the distal end of the instrument shaft, and two illustrative grasping jaws 24a,24b are shown held in distal clevis 26.
The amount of force in pitch (around axis 20) available at the distal tips 28a,28b is important for surgical tasks such as dissection and retraction in which one or both of the distal tips 28a,28b of the jaws 24a,24b are used to move or separate tissue. It can be seen that the relationship between the amount of force in pitch that the distal tips 28a,28b of the jaws 24a,24b can apply is directly related to (i) the amount of force that the cables can apply to move clevis link 26 in pitch, (ii) the length of the moment arm r1 in pulley portion 12 on which the cable in tension is acting, and (iii) the distance between the distal tips 28a,28b and the pitch axis 20 defined by the proximal clevis 22. To be effective, however, certain surgical instruments require long jaws, and so the amount of force available at the distal tips of such long end effectors is reduced to a level that makes the instrument relatively ineffective for some surgical tasks.
In the wrist architecture illustrated by FIGS. 2A and 2B, the amount of force the cables can apply to clevis link 10 is limited by the physical constraints of the cables or cable/hypotube combinations in the instrument. For example, above a certain tensile force, cables may have an increased tendency to break or to unacceptably stretch.
In addition, it is difficult to lengthen the moment arm r in the wrist architecture illustrated by FIGS. 2A and 2B. The instrument (e.g., about 8 mm outer diameter) must fit through a closely fitting cannula as it extends towards a surgical site within the patient (again, see FIG. 6), which places an upper limit on r1. Moreover, in the depicted wrist architecture, if r1 is increased, then the cables 18a, 18b begin to rub against the outer parts of the openings 32a,32b at the end of the shaft through which they run. This rubbing results in friction and stick/slip that causes, e.g., unacceptable cable wear and or hysteresis.
What is needed, therefore, is a wrist architecture that provides an increased force in pitch at the distal tip of a surgical end effector while conforming to an outer diameter limitation for the wrist mechanism due to existing surgical system size constraints.